This invention relates generally to tape systems for data storage and retrieval. More specifically, the invention relates to methods of improving data storage, and, specifically, the writing operation. The invention provides methods for minimizing off-track errors by monitoring the magnitude and/or timing of events that may possibly lead to off-track errors and taking corrective action.
Various computer data storage tape systems are well known. Tape systems are commonly used for data protection backup of computer files, and for dumping entire hard drive contents to a tape for backup and archiving. One category of such systems is the linear tape system, a subset of which subscribes to the xe2x80x9cLTO standardsxe2x80x9d (Linear Tape Organization standards). The LTO standards define requirements for hardware and software to enable universal use of standard tapes on LTO equipment produced by various manufacturers.
Schematics and information describing LTO technology are available in patent literature and at the website Itotechnology.com. A conventional LTO system is schematically portrayed in FIG. 1. LTO tape 10 is approximately xc2xd inch in width. Eight channel LTO tape has 384 tracks, readable and writeable by a head 12 with eight read/write elements 14. The eight read/write elements 14 in a conventional LTO system may simultaneously write or read eight tracks at a time all along the length of the tape 10, which is typically 600 meters long. After reaching the end of the tape 10, the head 12 is moved slightly, and the tape 10 is run in reverse. The position of the head 12 on the tape 10 is controlled in relationship to servo bands 16 on the tape that each contains six sub-bands. Also, the head 12 has two possible positions for each of the sub-bands within each of the servo bands. As a result, the tape has a total of (8xc3x974xc3x976xc3x972)=384 tracks for reading and writing.
Conventional linear tape systems have a maximum write and read speed, typically in the range of 2-8 meters/second (m/s), and, in the preferred embodiment herein, about 4.1 m/s. The linear tape travels at this speed through guides between two spools. Due to external vibrations, internal inconsistencies in tape, spool, or guide manufacture, and inconsistencies in the wrapping of tape around itself on the spool, the tape sometimes wanders transversely, relative to the writing head, so that centers of the tracks move transversely relative to the writing elements, that is, perpendicular to the length of the tape. This is called xe2x80x9cgoing off-track.xe2x80x9d These off-track events typically repeat themselves and frequently grow to a magnitude that results, if writing continues, in overwriting of another track or in the data not being found when it is to be read. Most conventional tape systems, therefore, are designed to react quickly when there is an xe2x80x9coff-track error,xe2x80x9d that is, when the head reaches an xe2x80x9coff-track limit.xe2x80x9d This off-track limit is predetermined, by the manufacturer/programmer, to be the limit between the area in which writing may be done properly and the area in which writing is not acceptable because of the possibility/probability of overwriting or unreadable writing. The off-track limit, in terms of microns transversely off of center-track, is set at different locations by various manufacturers, and is preferably a single off-track limit on each side of center in the range of about 10-20% of the track width. For a track having a 28-micron width, the off-track error limit may be set, for example, at about 3 microns, or another limit within the preferred 10-20% range.
The off-track sensing system of a conventional tape system cooperates with the closed-loop head-positioning servo control system. The head monitors its position relative to the servo bands, and so, in effect, monitors the position of the writing elements relative to the centers of the tracks. When the head reaches the off-track limit, meaning that the individual elements are off-track relative to their respective track centers, an error is signaled. The conventional tape system controller then turns off the writing function while the tape continues to travel at full speed, and waits until the head is back xe2x80x9con track,xe2x80x9d that is, within the acceptable position limits. Once the sensing system indicates that there is no longer an off-track error, writing is started again.
Each time an off-track error occurs, the process is repeated. If there are tape, tape spool, or vibration problems inherent in the tape system or in the environment in which the writing is being done, off-track errors may repeat frequently. Many off-track errors are caused by problems in the tape path such as the tape scuffing the reel flanges or bumps in the tape pack on the spool(s). These types of position error signal (PES) events occur at the rotational speed of one of the reels and the magnitude of the off-track error is very similar for each rotation. This type of off-track error, therefore, tends to repeat itself on each reel revolution, or at least very frequently, and, each time the error occurs, the process for xe2x80x9cretriesxe2x80x9d of writing, sometimes called the xe2x80x9cWrite With Off-Trackxe2x80x9d (WWOT) process, is repeated. The conventional tape system repeatedly shuts off the writing function, advances the tape until the off-track error ends, and starts writing again until another off-track signal occurs, all at full speed. If the off-track errors continue to interrupt writing for an extended period of time, the tape may travel a long distance without significant amounts of successful writing. Optionally, conventional retries may include repositioning the tape, that is, reversing the tape to back-up to a position on the tape where writing stopped, but such methods add complexity and waste time. To prevent endless unsuccessful attempts at writing, the conventional tape controller stops this process if a certain amount of writing has not been done successfully within a certain length of tape. Typically, if a data-set of typically 100-115 mm of data is not written in a total of 4 meters of tape, the controller will stop the process, without any more attempts, and signal a writing failure. The result of such off-track errors and the conventional WWOT process, therefore, reduces tape storage capacity and wastes time, and, occasionally, results in write failures.
Some tape speed control systems for linear tape have been developed that may adjust tape speed in order to match the speed of data coming from the host computer. Such systems prevent frequent stops and starts of the tape writing process when the tape speed is ahead of the data delivery speed from the host computer. In systems using such a host-matching speed control system, the speed at which writing, tape advancement, and retries are conventionally conducted may be the speed that is set in response to the host data transfer rate. In such systems, therefore, xe2x80x9cfull speedxe2x80x9d may not be the system""s highest possible speed, but rather the highest speed that is compatible with the host computer""s operation. In the Figures and hereafter, the term xe2x80x9cfull speedxe2x80x9d is used to denote either the maximum tape speed that is possible, or the maximum speed that is compatible with the host computer in systems that adjust speed to be compatible with the host computer. Patents that discuss speed control include U.S. Pat. No. 5,892,633 (Ayres, et al.); U.S. Pat. No. 5,764,430 (Ottesen, et al.); and U.S. Pat. No. 6,067,203 (Ottesen, et al.).
Still, there is a need for improved tape writing and reading performance. There is a need for reduction of off-track errors and write failures. The present invention addresses these needs.
The present invention comprises a method of reducing off-track errors in data storage tape systems. The present invention reduces tape speed in response to movement of the writing head/element off of the center of the track, prior to the writing head/element reaching an off-track error position. The present invention, therefore, preferably utilizes a double-limit system of an off-center limit in addition to an off-track limit, as a preventive measure to reduce off-track errors, write failures, and waste of time and tape. It is believed that speed reduction lessens the effect of such events as tape hitting a reel or bumps forming in the roll of tape, and, in doing so, lessens transverse relative movement of the tape and the writing head. Therefore, speed reduction helps the head positioning servo system improve control of the writing head position, and, thereby, helps prevent WWOT errors from occurring, especially the type of WWOT errors that start with off-center events growing in severity with every rotation of a tape reel. Preferably, the servo system position error signal (PES) is used to control the speed of the tape, in response to relative transverse movement of the head to a predetermined and preprogrammed off-center limit on either side of the track center, which is closer to the track center than the conventional off-track limits. Overall, the present invention tends to increase successful writing and to reduce failed writing.